Studies on a liquid crystal display have been developed as to the technique for controlling the intensity of light emitted from a backlight in accordance with a video signal in order to improve the contrast of a video picture to be displayed and to reduce power consumption. According to a general method, a screen is divided into a plurality of regions, and the intensity of a light source arranged in each region is separately controlled in accordance with a video signal.
However, the division number of the regions for which the intensity of the light sources is controlled is considerably smaller than the number of pixels of an input video picture, which causes a problem that a video picture having mixed bright and dark areas as in a night view suffers image deterioration such as uneven brightness and brightness variation due to the separate control by the light sources.
In a method suggested to avoid the image deterioration such as uneven brightness and brightness variation caused in the input video picture, a maximum luminance in divided regions for which the light sources are controlled (hereinafter referred to as illumination region) is calculated as light source intensity of the illumination region, and the light source intensity value is corrected to be increased so that the difference in the light source intensity between each illumination region and its adjacent illumination region becomes a permissible value or smaller.
However, in this method, the intensity of each illumination region is corrected to be increased in accordance with the maximum light source intensity value in all illumination regions and thus the intensity of the illumination regions becomes totally high to reduce uneven brightness, which leads to a problem that light emitted from the light sources becomes wastefully bright. Further, the threshold value for the intensity difference between the illumination regions adjacent to each other should be made larger in order to reduce power consumption, which leads to a problem that uneven brightness and brightness variation cannot be sufficiently reduced.